1. Field of the Invention
The present invention relates to a ceramic circuit board which is constructed by a ceramic substrate and metal circuit plates attached to both surfaces of the ceramic substrate.
The invention also relates to a ceramic circuit board which is constructed by a ceramic substrate, metal circuit plates attached to both surfaces of the ceramic substrate, and a metal column arranged within the ceramic substrate to connect the two metal circuit plates, and to a method for manufacturing said ceramic circuit board.
2. Description of the Related Art
In recent years, as circuit boards such as power module boards and switching module boards, ceramic circuit boards have come into common use. The examples thereof include: a ceramic circuit board constructed by bonding a metal circuit plate made of copper or the like to a metallized layer coated onto a ceramic substrate, via a brazing material such as a silver-copper alloy; a ceramic circuit board constructed by directly bonding a metal circuit plate made of copper or the like onto a ceramic substrate, via an active metal brazing material obtained by adding titanium, zirconium, hafnium, or its hydride to a silver-copper eutectic alloy; and a ceramic circuit board constructed in accordance with the so-called DBC (Direct Bond Copper) method, in which a copperplate is placed onto a ceramic substrate, and the ceramic substrate and the copper plate are directly bonded together by application of heat.
In one aspect of the prior art, in those ceramic circuit boards, in order to increase the mounting density of the metal circuit plate, the metal circuit plate is bonded both on the upper surface and on the lower surface of the ceramic substrate, and the upper and lower metal circuit plates are electrically connected to each other by a metal column arranged within a through hole formed in the ceramic substrate.
Among the aforementioned ceramic circuit boards, for example, the one constructed by directly bonding a metal circuit plate made of copper, etc. onto a ceramic substrate via an active metal brazing material is fabricated as follows. Firstly, there is prepared a ceramic substrate made of an electrically insulating material such as sintered aluminum oxide, sintered aluminum nitride, sintered silicon nitride, or sintered mullite. The ceramic substrate has a through hole drilled therethrough in the thickness direction. Secondly, a metal column is arranged within the through hole, and a brazing material paste, which is obtained by mixedly adding an organic composition or solvent to silver brazing filler powder (powder of silver-copper alloy), is applied to each end face of the metal column. Then, a metal circuit plate of predetermined pattern is placed in abutment with the ceramic substrate, with a brazing material sandwiched therebetween, such as an active metal brazing material obtained by adding at least one of titanium, zirconium, hafnium, and their hydrides to a silver-copper alloy. Lastly, the assembly thus obtained is heated in a reducing atmosphere at a temperature of approximately 900° C. to melt the brazing material paste and the brazing material. Thereby, the metallized layer and the metal circuit plate are bonded together via the active metal brazing material, and also the metal circuit plate and the metal column are bonded together via a brazing material such as a silver brazing filler.
The ceramic circuit board thus fabricated is, after mounting thereon an electronic component such as a semiconductor device, for example an IGBT (Insulated Gate Bipolar Transistor) or an MOS-FET (Metal Oxide Semiconductor-Field Effect Transistor), using an adhesive such as solder, assembled into a resin case having an integrally-molded external input/output terminal, thereby realizing a semiconductor module. This semiconductor module finds a wider range of applications involving industrial equipment such as a robot, an actuator of an electric train, an electric vehicle, and the like, and is therefore required to operate with a high degree of reliability under harsh environments.
The conventional ceramic circuit boards, however, have the following disadvantages. If the length of the metal column is greater than the thickness of the ceramic substrate, a brazing material is absent in between the metal column and the metal circuit plate, or it is present little in quantity, if any. In this state, the metal column and the metal circuit plate are connected to each other with poor reliability, with the result that an electronic component such as a semiconductor device, which is mounted on the metal circuit plate, is unable to operate normally with stability. Moreover, if the metal column is made unduly long, it thrusts up the metal circuit plate, and consequently part of the metal circuit plate located near the metal column rises and becomes out of touch with the ceramic substrate. This causes imperfect bonding of the metal circuit plate to the ceramic substrate, resulting in the ceramic circuit board being poor in reliability.
By contrast, if the length of the metal column is smaller than the thickness of the ceramic substrate, it is difficult to supply an adequate amount of a brazing material paste, which is print-coated on the upper and lower surfaces of the metal column. If the brazing material paste is in short supply, the metal column and the metal circuit plate cannot be connected to each other by the brazing material, with the result that the ceramic circuit board is brought into an electrically opened state and thus fails to function properly. Even if the metal column can be connected to the metal circuit plate, the brazing material portion existing therebetween is so thin that it sustains high continuity resistance. Consequently, heat is generated locally at the time of energization, and this interferes with the proper and stable operation of an electronic component, such as a semiconductor devices, which is mounted on the ceramic circuit board.
In another aspect of the prior art, in those conventional ceramic circuit boards, in order to increase the mounting density of the metal circuit plate, the metal circuit plate is bonded both on the upper surface and on the lower surface of the ceramic substrate, and the upper and lower metal circuit plates are electrically connected to each other by a brazing material filled in a through hole formed in the ceramic substrate.
The aforementioned ceramic circuit boards, for example, the ceramic circuit board constructed by bonding a metal circuit plate made of copper, etc. to a metallized layer coated onto a ceramic substrate via a brazing material is fabricated as follows. Firstly, there is prepared a ceramic substrate made of an electrically insulating ceramic material such as sintered aluminum oxide, sintered aluminum nitride, sintered silicon nitride, or sintered mullite. The ceramic substrate has, on each of its upper and lower surfaces, a metallized layer of predetermined pattern, and also has a through hole drilled therethrough in the thickness direction. Secondly, the through hole of the ceramic substrate is filled with a brazing material paste obtained by mixedly adding an organic composition or solvent to silver brazing filler powder (powder of silver-copper alloy). Then, a metal circuit plate of predetermined pattern is placed in abutment with the metallized layer, with a brazing material such as a silver brazing filler sandwiched therebetween. Lastly, the assembly thus obtained is heated in a reducing atmosphere at a temperature of approximately 900° C. to melt the brazing material paste and the brazing material. As a result, the brazing material, such as a silver brazing filler, serves to bond together the metallized layer and the metal circuit plate, and also bond together the upper and lower metal circuit plates of the ceramic substrate.
This conventional ceramic circuit board, however, poses the following problem. In the construction, the two metal circuit plates bonded to the upper and lower surfaces of the ceramic substrate are electrically connected to each other by the brazing material filled in the through hole formed in the ceramic substrate. Furthermore, the through hole formed in the ceramic substrate is filled with a brazing material paste obtained by mixedly adding an organic composition or solvent to silver brazing filler powder (powder of silver-copper alloy), and subsequently heating treatment is performed thereon at a temperature of approximately 900° C. In this case, air existing between the individual silver brazing filler particles is contained in large quantity in the molten silver brazing filler, and the brazing material accordingly has a porous structure and its continuity resistance is as high as 7 to 10 μΩ·cm in terms of resistivity. Thus, in the conventional ceramic circuit board, when large electric current exceeding 10 A flows through the metal circuit plate and the brazing material filled in the through hole, the brazing material portion filled in the through hole undergoes resistive heat generation, and the resultant heat undesirably acts upon an electronic component, such as a semiconductor device, which is bond-fixed onto the metal circuit plate via an adhesive such as solder. As a result, the electronic component cannot be operated stably under an unduly high temperature.